How important are individuals?

Do you believe that individuals may have a great effect upon the course of science and history in general as proposed by the 'Great man' theory? Or do you subscribe to Tolstoy's views that history is shaped by events that would have come about in any case regardless of the existence of a few people?

For example, would physics have developed in much the same way and at a similar rate if it were not for the likes of Einstein and Newton or even less glamorous geniuses such as Dirac and Maxwell? Personally, I believe it would have, and that any delay would have lasted only a few years to a decade or so at the most. I believe this because all scientific work is building upon the work of others, I have never heard of someone developing an entirely new field and line of research entirely by themselves and as such it would appear that if it were not that individual that made the key discovery or insight it would simply have been someone else.

The reason I believe the delays would be so short is because of the number of times that things have been discovered or worked out independently yet almost simultaneously. For example, the invention of the calculus by Newton and Leibniz or the development of radio by Tesla and Marconi.